DoT: remove TLS-related kludge in isc__nmsocket_connecttimeout_cb()

DoT: remove TLS-related kludge in isc__nmsocket_connecttimeout_cb()

This commit ensures that 'sock->tls.pending_req' is not getting
nullified during TLS connection timeout callback as it prevents the
connection callback being called when connecting was not successful.

We expect 'isc__nm_failed_connect_cb() to be called from
'isc__nm_tlsdns_shutdown()' when establishing connections was
successful, but with 'sock->tls.pending_req' nullified that will not
happen.

The code removed most likely was required in older iterations of the
NM, but to me it seems that now it does only harm. One of the well
know pronounced effects is leading to irrecoverable zone transfer
hangs via TLS.